When Joe’s in the cart, perhaps, it’s serious (6)
I believe the answer is:
tragic
'when joe's' is the definition.
I can't tell whether this defines the answer.
'in the cart perhaps it's serious' is the wordplay.
'in the' indicates putting letters inside.
'perhaps' is an anagram indicator.
'it's serious' becomes 'gi' (I can't explain this - if you can you should believe this answer much more).
'cart' with letters rearranged gives 'trac'.
'trac' enclosing 'gi' is 'TRAGIC'.
Can you help me to learn more?
(Other definitions for tragic that I've seen before include "like King Lear" , "Dreadful" , "Terribly sad and distressing" , "Calamitous, causing great sadness" , "Ill-fated" .)